Process for coating metals



3 tallic articles, especially to iron and steel.

Patented June 29, 1937 PATENT OFFICE PROCESS FOR COATING METALS Floyd Francis Oplinger, Niagara Falls, N. Y., as-

signor to E. I. du Pont de Nemours 86 Company, Wilmington, Del., a corporation of Delaware No Drawing. Application May 21, 1935,

Serial No. 22,651 A 13 Claims.

This invention relates to the production of rust-proof coatings on iron and steel and other metals and has for an object an improved method for applying tin and tin alloy coatings to me- A further object of the invention is to provide a means for transforming a relatively dull, electroplated tin or tin alloy coating to a bright, smooth, mirror-like coating. A further object is to produce 3 bright, smooth -coatings of electroplated tin on cold-rolled steel. Other objects will be apparent from the following description.

In my copending application Ser. No. 709,171, I have described and claimed a method for 5 producing bright tin coatings on iron, steel and other metals, which method comprises first electroplating with tin, thereafter heat-treating the electroplated surface under substantially nonoxidizing conditions to fuse the tin electrodeposit and then cooling. This method, which is applicable to tin, fusible tin alloys and other fusible metallic electrodeposits, transforms the relatively dull electroplated coatings to bright, smooth, mirror-like surfaces which are highly 3 corrosion resistant because the heat-treating operation effectively closes any pin holes, pores or crevices which may occur in the electrodeposit. In the said copending application I have disclosed that this method when applied to coldrolled sheet steel does not produce the desired smooth appearance of the finished heat-treated article unless the steel is specially treated prior to electroplating by a method disclosed and claimed in said application. This method of pretreating cold-rolled steel to cause the finished product to be smooth and bright rather than rough and lumpy consists in first treating the steel surface with an acid to such extent that the surface is distinctly and visibly etched and fol- 0 lowing this treatment with an alkaline treatment, e. g. anodic treatment in an alkaline bath, preferably one containing an alkali metal cyanide.

I have now discovered that by the employment 3 of the herein described invention I may produce smooth, bright coatings of tin or tin alloy on cold-rolled steel and the like by first electroplating with the tin or tin alloy followed by heat treatment to the melting point of the electro- 0 deposit under substantially non-oxidizing condi- 5 compound of one or more of the following metals (Cl. 204-17l to the tin or tin alloy electroplating bath: copper, nickel, silver or a cobalt, thereby simultaneously plating out a small amount of the metal whose compound has been added to the bath together with the tin or tin alloy electrodeposit. In arriving at this discovery, I have testeda number of various metals for this purpose and have found that only the metals enumerated above are suitable for accomplishing the desired result. For example, metals such as lead, cadmium, zinc and mercury are not suitable for use in the present invention. The presence of such metals does no harm as a rule, but in order to obtain smooth, bright coatings of tin or tin alloy by heat-treating the electrodeposit on cold-rolled steel, I have found it essential that at least one of the metals: copper, nickel, silver or cobalt be present in a small amount in the electrodeposit. By this means I have produced smooth, bright mirrorlike coatings of tin upon various types of coldrolled sheet steel without using the above described acid etch-alkaline pre-treatment prior to electroplating steel, the steel prior to electroplating being merely cleaned by any known process for removing grease, dirt and the like prior to electroplating. If desired, in practicing my invention the acid etch-alkaline pre-treatment may be used but this is not essential and ordinarily is not especially advantageous when practicing the herein described invention.

In one method of practicing the herein described invention, I may add to an ordinary tin electroplating bath a compound of copper, nick'- el, cobalt or silver soluble in the bath and electroplate the article to be coated in the bath, whereby an electrodeposit is obtained which contains a relatively small amount of the metal whose compound is added to the bath. Prior to electroplating, the articleto be electroplated may be cleaned by any desired method to remove grease, dirt, oxide and the like. 'If an acid is used to remove oxide or scale, it is not necessary to continue the acid treatment to the point where visible etching occurs. When the desired thickness of electrodeposit has been obtained, the electroplated article is rinsed, dried, and then heated under substantially non-oxidizing conditions to a temperature slightly above the melting point of electrodeposit. The various methods of heat-treating under non-oxidizing conditions which are described in my aforesaid copending application are suitable for this step. For example, the plated article may be immersed in hot oilor molten tallow, heated in an atmosphere of substantially non-oxidizing gas such as r pose.

hydrogen, nitrogen, carbon vapor, or immersed in a molten salt bath. If desired, in these methods, electrical induction or electrical conduction may be used to heat the electroplated metal. Preferably, the heat-treating is not prolonged much beyond the time required to obtain complete fusion of the electrodeposit and then is preferably cooled under conditions least favorable to oxidation, e. g. by quenching in cold water or cold oil.

In practicing my invention, I prefer to use copper or nickel or both of these metals since these metals are less expensive than silver and cobalt and generally are more satisfactory for my pur- By experimentation I have found that it is necessary to plate out relatively small amounts of copper or nickel on the cathode in order to produce the desired smooth, bright, mirror-like coats on subsequent heat-treatment. For example, in order to produce the desired result, the tin electrodeposit should contain not less than about 1.4% of copper, or silver or not less than about 0.4% of nickel or cobalt. If the electrodeposit contains substantially less than these amounts, the desired result usually is not obtained and on subsequent heat-treating the coating tends to be rough or lumpy and does not appear to adhere well to the underlying surface. It is to be understood that said lower limits of copper or nickel suitable for practicing my invention are based on the specific methods of analysis which are described hereinafter in the examples given to illustrate my invention, which methods comprise an electrolytic method for determining copper and the use of dimethyl glyoxime for the determination of nickel. While generally satisfactory results may be obtained by plating out copper or nickel in the tin electrodeposit at or near to the lower limits above specified, I usually prefer to use some higher amounts of such metals to insure consistent results in extended periods of operation. Thus I prefer to operate so that the copper content of the electrodeposit is not less than about 2% by weight or, if nickel is used instead of copper, so that the nickel content in the electrodeposit is not less than about 0.5% by weight. The melting point of the resulting electrodeposit obviously will depend upon the amount of copper or nickel content, the effect of large proportions of these metals being to raise the melting point of the electrodeposit. I prefer to adjust the copper or nickel content of the bath so that the melting point of the resulting composite electrodeposit is not greater than about 260 C. at or below which temperature the heattreating operation may be conveniently carried out in a molten tallow bath. I have found that in order to obtain this result, the copper content of the electrodeposit should not exceed about 15% or the nickel content, if nickel is used, should not exceed about 2% by weight. By operating in this range, I have found that it is possible to obtain the desired results by heat-treating the electrodeposit under non-oxidizing conditions in a temperature range of about 230260 C.

My invention is further illustrated by the following examples:

Example 1 A plating solution was prepared, containing:

1 OZ. per gal. Sodium stannate 20 Sodium hydroxide 1.5 Sodium acetate 2 days for l-hour periods as shown in the table below. During each entire electroplating period, weighed cathodes of pure tin foil were plated in the bath and 2 to 3 cold-rolled sheet steel cathodes were simultaneously plated for 10 minutes each. The plated tin foil cathodes were analyzed for nickel by the following procedure: The elec troplated foil is washed, dried and weighed to determine the weight of the electrodeposit. The foil then is dissolved in a solution of 10 parts of concentrated nitric acid, 10 parts of concentrated hydrochloric acid and 25 parts of'water. About 25 grams of sodium tartrate is added to the resulting solution to prevent precipitation of tin and the solution is made slightly alkaline with ammonium hydroxide. Nickel then is precipitated with dimethyl glyoxirne and the precipitate filtered off, washed, dried and weighed in the usual manner for the determination of nickel. Preferably, the precipitate is redissolved in hydrochloric acid, sodium tartrate'added and the nickel re-precipitated as described above. The analytical results are shown in the following table:

Day Time Analysis of deposit T r y 9:45-10:45 A. M. Fnday {2:0o-3;00 r. M.

, 10:0041110 A. M. Monday {memo r. M.

9:50-10:50 A. M. Tuesday l2:00-l:0() P. M.

The electroplated steel cathodes were washed, dried and then heat-treated by immersing them for 10 to 30 seconds in molten tallow maintained at a temperature of about 250 C. and cooled in the air. In each trial, the resulting heat-treated plate was smooth, bright and mirror-like.

Example 2 A tin plating solution was made containing the following ingredients:

Gms./L. Sodium stannate Sodium acetate 15 Sodium hydroxide 11 the above mentioned conditions for 15 minutes and at the same time a weighed sheet of pure tin foil was also electroplated for the same period of time. The sheet steel cathodes, after electroplating, were washed, dried and then heat-treated at 230-260 C. by immersing in a heated bath of molten tallow. The electroplated tin foil was analyzed for copper content by the following method: The plated foil was Washed, dried and weighed and then was dissolved with a large excess of 50% nitric acid solution and the solution evaporated down to a dry powder at a temperature of 100 C. The resulting powder was treated with 25 cc. of concentrated nitric acid and 75 cc. of water and heated to approximately 80 C. and the hot solution was filtered through No. l filter paper. The precipitate on the filter was thoroughly washed with 1% nitric acid to remove all traces of copper and the washings combined with the filtrate. The resulting clear solution was electrolyzed, using platinum gauzes as anode and cathode with 2 amperes of current. Electrolysis was continued until there was no further gain in weight of the cathode. The cathode then was dried and weighed to determine the amount of copper in the tin electrodeposit. The results obtained are shown in the following table:

A tin plated bath was prepared having the following composition:

Oz./gal. Sodium stannate 20 Sodium acetate 2 Sodium hydroxide 1.5

Various amounts of a sodium-copper cyanide solution of known copper concentration were added to the bath from time to time and after each addition cold-rolled sheet steel cathodes were electroplated in the bath, using Straits tin anodes and a cathode current density of 15 amperes per square foot. At the same time sheets of tin foil were electroplated with the same cathode current density. The electroplated steel cathodes were heat-treated as described in Example 3 while the plated tin foil samples were analyzed for their copper content, whereby the copper content in the electrodeposit was determined. All of the electrodeposits (except that of Sample 8 'which contained 23.6%) were transformed to smooth, bright, mirror-like coatings by the heattreatment. In the case of Sample 8, the melting point of the electrodeposit, because of the high copper content, was above 260 C. and therefore the heat treatment failed to fuse the coating. The results obtained are shown in the following table:

a, i (11 content (.u(N content of deposit of hath Percent Gm. 7m liir'r Example 4 Cold-rolled sheet steel was electroplated at 15 amperes per square foot cathode current density, from a tin plating bath having the composition of the bath of Example 1, except that 2 gms. per liter of a silver cyanide (dissolved in sodium cyanide'solution) was added in place of nickel cyanide. The electroplated steel, on heating in molten tallow at 245 C. readily fused, forming a smooth coating.

Example Cold-rolled steel was electroplated as in Example 4, except that in place of silver cyanide, sodium cobaltocyanide was added to the bath in an amount equivalent to 4.5 gms. per liter of cobalt. On heat-treating in molten tallow, a bright, smooth tin coating was obtained. The tin electrodeposit was found by an analytical method similar to that of Example 1 to contain 0.35% of cobalt. Similar trials, using smaller amounts of cobalt, produced rough, lumpy deposits on heattreating at 245 C.

In practicing my herein described invention, I prefer to add to the electrolyte an amount of soluble compound of copper, nickel, silver and/or cobalt not less than about 0.01 gram per liter,"

in order to insure that there always will be sufficient of the added metal in the bath to plate out I added copper or nickel compound and thus decrease its concentration. To avoid concentration of the added metal compound being thus unduly decreased, I prefer to maintain its concentration in the bath not lower than 0.01 gram per liter and preferably 0.05 to 1.5 grams per liter or higher. For example, when operating by the addition of sodium-copper cyanide to an alkaline tin plating bath, I prefer to maintain the copper cyanide concentration in the bath at from 0.05 to 1.5

grams per liter and if operating by the addition of dissolved nickel cyanide I prefer to maintain the nickel cyanide concentration at between 0.1 to 0.4 gram per liter. In ordinary plating operations, the required concentration of the .added metal to the electroplating bath may readily be maintained by periodical additions of the metal compound at periodic intervals, e. g. once or twice daily. When starting the operation of a freshly made tin electroplating bath in accordance with my invention, I prefer to allow the elec trolyte to settle or to filter it before the salt of copper, nickel, etc. is added thereto.

My herein described invention is not restricted to the above described specific methods of operation which have been set forth by way of example. For example, instead of adding copper, nickel, cobalt and/or silver compound to the tin plating bath in order to obtain an electrodeposit containing such metal, I may electroplatc alternate layers of tin and the added metal from separate plating baths. Thus I may electroplatc a thin layer of copper or nickel on an electrodeposited tin layer and subject the resulting electrodeposit to the heat-treating operation, with satisfactory results. It is preferable, however, to simultaneously plate the tin and added metal from a single electrolyte, since this generally results in better distribution of the added metal throughout the coating layer.

My invention, while particularly adapted for coating cold-rolled sheet steel and similar coldworked ferrous metal articles, is equally useful for plating other ferrous metal articles, e. g. hotrolled steel sheet, steel wire and the like and nonferrous metals having sufficiently high melting points to withstand the required heat-treatment, e. g. copper, brass, bronze and the like. My method results in smooth, mirror-like coatings of high corrosion resistance on the various metals to which it may be applied. The only preparatory treatment required prior to the electroplating step is the usual treatment for removing oxide, grease and dirt which is ordinarily used in electroplating operations.

I claim:

1. A process for coating a cold rolled steel surface, comprising electroplating said surface with a tin alloy consisting preponderantly of tin and containing at least one metal selected from the group consisting of copper, nickel, silver and cobalt and thereafter heating the electroplated surface under substantially non-oxidizing conditions to a temperature above the melting point of the electrodeposit and then cooling.

2. A process for coating cold-rolled steel comprising electroplating said steel from a tin bath containing not less than about 0.01 gram per liter of a dissolved copper compound and thereafter heating the electroplated steel under substantially non-oxidizing conditions to a temperature above the melting point of the electrodeposit and then cooling.

3. A process for coating cold-rolled steel comprising electroplating said steel from a tin bath containing not less than about 001 gram per liter of a dissolved nickel compound and thereafter heating the electroplated steel under substantially non-oxidizing conditions to a temperature above the melting point of the electrodeposit and then cooling.

4. A process for coating cold-rolled steel comprising electroplating said steel from an alkaline tin plating bath containing about 0.05 to 1.5 grams per liter of dissolved cuprous cyanide and thereafter heating the electroplated steel under substantially non-oxidizing conditions to a temperature above the melting point of the electrodeposit and then cooling.

5. A process for coating cold-rolled steel comprising electroplating said steel from an alkaline tin bath containing about 0.1 to 0.4 grams per liter of dissolved nickel cyanide and thereafter heating the electroplated steel under substantially non-oxidizing conditions to a temperature above the melting point of the electrodeposit and then cooling.

6. A process for producing a bright, smooth coating of tin on cold-rolled steel comprising simultaneously electroplating the surface of said steel with tin and another metal selected from the group consisting of copper, nickel, silver and cobalt in such manner that the resulting electrodeposit consists preponderantly of tin and contains an amount of said other metal not less than about 0.4% by weight when said other metal is nickel or cobalt and not less than about 1.4% when said other metal is copper or silver and thereafter heating the electroplated steel under substantially non-oxidizing conditions to a temperature above the melting point of said electrodeposit.

7. A process for producing a bright, smooth coating of tin on cold-rolled steel comprising simultaneously electroplating said steel with tin and copper in such manner that the resulting electrodeposit consists preponderantly of tin and contains not less than about 1.4% by weight of copper and thereafter heating the electroplated steel under substantially non-oxidizing conditions to a temperature above the melting point of said electrodeposit.

8. A process for producing a bright, smooth coating of tin on cold-rolled steel comprising simultaneously electroplating said steel with tin and copper in such manner that the resulting electrodeposit consists preponderantly of tin and contains about 2 to 15% by weight of copper and thereafter heating the electroplated steel under substantially non-oxidizing conditions to a temperature of about 230 to 260 C.

9. A process for producing a bright, smooth coating of tin on cold-rolled steel comprising simultaneously electroplating said steel with tin and nickel in such manner that the resulting electrodeposit consists preponderantly of tin and contains not less than about 0.4% by weight of nickel and thereafter heating the electroplated steel under substantially non-oxidizing conditions to a temperature above the melting point of said electrodeposit.

10. A process for producing a bright, smooth coating of tin 0n cold-rolled steel comprising simultaneously electroplating said steel with tin and nickel in such manner that the resulting electrodeposit consists preponderantly of tin and contains about 0.5% to 2% by weight of nickel and thereafter heating the electroplated steel under substantially non-oxidizing conditions to a temperature of about 230 to 260 C.

11. An article of manufacture comprising cold rolled steel coated with a fused metal coating which is preponderantly of tin and which contains another metal selected from the group consisting of copper, nickel, silver and cobalt, said coating containing not less than about 0.4% by weight of said otherrnetal if said other metal is nickel or cobalt and not less than about 1.4% by weight of said other metal if said other metal is copper or silver.

12. An article of manufacture comprising coldrolled steel coated with a smooth, bright, fused electrodeposit which is preponderantly of tin and which contains 1.4 to 15% by weight of copper.

13. An article of manufacture comprising a cold-rolled steel surface coated with a smooth,

bright, fused electrodeposit which is preponderantly of tin and which contains 0.4 to 2% by Weight of nickel.

FLOYD FRANCIS OPLINGER. 

